1. Field of the Invention
The present invention relates to an automatic frequency control method and device, which are applied to digital demodulation processing in satellite communication, mobile satellite communication, mobile terrestrial communication, and a demodulator to which said device is applied.
2. Description of the Related Art
Recently, in satellite communication, mobile satellite communication, and mobile terrestrial communication, digital modulation and demodulation have been actively researched. In particular, in the environment of mobile communications, a radio signal is greatly influenced by fading. Therefore, various demodulation systems which stably operate even in such a fading environment have been considered. Among these systems, a system, which is constructed so that absolute coherent detection can be executed even in a fading environment by estimating and compensating fading distortion by using known signals, has been recognized. In the case where fading distortion is estimated and compensated after quasi-coherent detection or the like is carried out by this system, to estimate and compensate fading distortion with high accuracy, it is necessary that the frequency offset between the carrier wave frequency of a radio transmission signal and the oscillation frequency of a reference signal for quasi-coherent detection is small.
However, in the case where the frequency stability and accuracy of the oscillation circuit of a transmitter-receiver are insufficient, unless the frequency of a radio receiving signal is automatically controlled by eliminating this frequency offset by some processing, fading distortion cannot be estimated and compensated with high accuracy.
In mobile communication, transmission and receiving are carried out between a fixed station and a mobile station or between mobile stations. Therefore, when two stations relatively move, the frequency of a radio receiving signal deviates due to Doppler variation. Therefore, even if the stability and accuracy of the oscillation circuit of the transmitter-receiver are good, frequency offset occurs between the frequency of the radio receiving signal and the oscillation frequency of the reference signal.
A technique for compensating frequency offset is disclosed in, for example, Japanese Patent Laid-Open No. 93302/1997 titled “DIGITAL MOBILE RADIO COMMUNICATION METHOD”. By the conventional technique disclosed in this document, frequency offset is eliminated by using phase fluctuation information of known signals (pilot signals).
In this conventional technique, a radio transmission signal into which two-symbol known signals have been inserted every inserting period NF from the transmission side is transmitted. On the other hand, at the receiving side, the phase change amount between the sequential two-symbol known signals is calculated, and in accordance with the calculated phase change amount, the phase of the radio receiving signal is rotated. Frequency offset is thus eliminated from the radio receiving signal.
In electric wave transmission channels, there is a Rician fading transmission channel in which direct waves and multi-path waves are mixed. In this case, the direct wave is Doppler-shifted. Therefore, the frequency fD of the direct wave further deviates by Doppler-shift amount fDP caused by the Doppler-shifting from the offset fOFST caused by stability of the oscillation circuit as shown in FIG. 24A.
On the other hand, in the abovementioned prior art, the phase change amount between the sequential two-symbol known signals is supposed as frequency offset. The frequency offset calculated in this case is equivalent to the difference between the oscillation frequency fO at the receiver side and the frequency fD of the direct wave. That is, in the prior art, as shown in FIG. 24B, frequency control is carried out so that the frequency fD of the direct wave almost coincides with the oscillation frequency fO. In this case, the center frequency fM of the Doppler spread deviates from the oscillation frequency fO by the Doppler-shift amount fDP. Therefore, the Doppler spread apparently further broadens, so that the frequency corresponding to the end of the Doppler spread greatly deviates from the oscillation frequency fO. Accordingly, frequency offset cannot be satisfactorily compensated. Therefore, bit error rate characteristics (hereinafter, referred to as BER characteristics) after the radio receiving signal is demodulated may deteriorate.